1. Field of the Invention
The present invention relates to an electron beam projection apparatus which produces an auxiliary magnetic field within a convergent magnetic field and adjusts a focal position of a convergent electron beam by changing the intensity of the auxiliary magnetic field. This focal adjustment apparatus is normally called a refocusing lens, therefore, this term is used in the following.
2. Description of the Related Art
The use of an electron beam projection system is well-known as a method for forming minute patterns on large scale integrated circuit semiconductor devices. As the integration density and complexity of a circuit increases, conventional optical lithographic techniques encounter a limit to the production of extremely dense circuit patterns. As a result, another form of pattern-making technique with higher accuracy, such as electron beam lithography, is required. The primary advantage of electron beam lithography is its high resolution capability. Problems due to diffraction effects which are inherent in optical lithography, are resolved by electron beam lithography because the equivalent wavelength of electrons in the 10 to 20 kilovolt energy range is less than 1 angstrom which is substantially smaller than that of an ultraviolet ray.
The electron beam projection apparatus provides an electron source, a beam deflection system in which the beam is deflected in a raster or vector scanning manner, and a magnetic object lens by which the beam is converged onto the object. The electron beam projection apparatus further provides a beam-forming system in which electrons emitted from the electron source are formed into a fine beam having a round or rectangular crosssection. The shape of the electron beam is changed according to the projection patterns on an object. This shaping is performed by passing the electron beam through an aperture arranged in a path of the beam. When the area size of the aperture is changed, the current intensity of the electron beam changes and the focal position also moves in an axis direction of the magnetic object lens. The adjustment of the focal position can be performed by changing the current intensity of the magnetic object lens. However, the response time of the magnetic object lens for changing the focal length is relatively long because the number of turns on a coil of the magnetic object lens is large and a housing of this magnetic object lens is composed of magnetic material. Because the change of the shape of the electron beam is carried out during the pattern projection, the adjustment speed of the focal position is required to be very high. Consequently, the magnetic object lens is not adequate for adjusting the focal position movement due to the change of the beam shape. The refocusing lens is used for this adjustment.
The refocusing lens is arranged within a convergent magnetic field produced by the magnetic object lens and produces an auxiliary magnetic field overlapping the convergent magnetic field of the object lens. The refocusing lens is composed of a coil and its housing. The coil is required to be formed with non-magnetic material such as copper, and the housing is required to be formed with non-magnetic and non-conductive material such as ceramics. The intensity of magnetic field produced by the refocusing lens is extremely smaller than that of the magnetic object lens, for example, 1/1000. The refocusing lens is desirably arranged at the center in the axis direction of the convergent magnetic field produced by the magnetic object lens, and axes of two magnetic fields are required to agree with each other. When the refocusing lens is not arranged at the center in the axis direction of the convergent magnetic field, the refocusing lens cannot efficiently change the focal position of the electron beam. When the axes of two magnetic fields do not agree, the projection position of the electron beam on the object moves according to the intensity fluctuation of the auxiliary magnetic field, and this position movement of the electron beam causes errors in the projection patterns.
As described in above, the refocusing lens is arranged within the convergent magnetic field, that is, within the magnetic object lens. The magnetic object lens provides a small circular bore at the center which the electron beam passes through, and the refocusing lens is required to be arranged within this circular bore. Since the refocusing lens only includes the refocusing coil and does not include a core such as a pole piece which is provided in the magnetic object lens for producing a strong and precise convergent magnetic field, the configuration of the auxiliary magnetic field produced by the refocusing lens cannot be produced so precisely as that of the object lens. Therefore, position and inclination of the refocusing lens opposite to the magnetic object lens are required to be adjusted when the apparatus is fabricated. As described later, the adjustment of the axis position in the direction perpendicular to the axis can be equivalently replaced by the inclination adjustment.
For performing the above adjustment, the conventional apparatus includes an inclination adjustment mechanism in which a movable plate is supported by three points and the inclination of the movable plate is adjusted by changing two positions of the three supporting points. The refocusing lens is fixed to the movable plate of this inclination adjustment mechanism and is suspended into the bore of the magnetic object lens. In an adjustment operation, the inclination of the refocusing lens is adjusted while changing the current of the refocusing coil, and the adjustment operation is completed when the position of the electron beam on the object does not move although the current of the refocusing coil fluctuates.
In the inclination adjustment mechanism used in the conventional apparatus, inclination changes produced by moving supporting points are not independent of each other because the movable plate is supported at three points, therefore, adjustment operations become very complex.
Further, in the inclination adjustment mechanism, the refocusing lens moves in the axis direction according to the change of the plate inclination because the refocusing lens is suspended from the movable plate. When the refocusing lens moves in the perpendicular direction, the focal position of the electron beam moves. In order to adjust this movement of the focal position, the current intensity of the refocusing coil is adjusted. As described above, the efficiency of the refocusing lens contributing to the change of the focal position is maximum when the refocusing lens is arranged at the center of the convergent magnetic field. Therefore, if the refocusing lens moves in the axis direction according to the adjustment of the inclination, there is a problem that the efficiency of the refocusing lens decreases and the current intensity of the refocusing coil becomes large.